The present invention relates to a radioactive gas monitoring device and in particular to a device that is capable of providing a continuous integrated readout of radioactive gas exposure.
Instruments for measuring radioactive gases are well known and widely used. However, gas monitoring instruments currently available, especially those for measuring radioactive gas concentrations in air, provide only a reading of instantaneous gas concentration. In other words, these instruments are only adapted to indicate the level of radioactive gas concentration present at the time immediately preceding the measurement. While such instruments are therefore of value in determining the existence of gross radioactive gas leakage, they do not provide precise information concerning the total accumulated radioactivity which personnel in a particular area might be exposed to over an extended period of time. Such information is particularly useful in view of the current NRC regulations governing the permissible levels of radioactive gas exposure which speak in terms of a Maximum Permissible Concentration (MPC) over a period of 40 hours, to cover individual exposure during one calendar week. Thus, the regulations do not permit exposure of more than 40 MPC-HOURS in any one week.
Accordingly, it can be appreciated that it is desirable to provide a radioactive gas monitoring instrument that is capable of measuring total integrated activity over an extended period of time. Hence, the present invention, while also indicating instantaneous radioactive gas concentrations, integrates the total exposure to provide a readout in units which can be directly correlated to MPC exposure levels established by the federal government.
As will subsequently be described in greater detail, the preferred embodiment of the present invention is adpated to measure concentrations of Xe-133 in room air or Xe-133 absorption trap effluent, although it will be appreciated that the present invention can be readily adapted to measure other radioactive gases as well. Xe-133 emits a beta particle which is detectable by a Geiger tube. Actual test chambers have shown that a Geiger tube will count at a rate of approximately 300 counts per minute when the concentration of Xe-133 is equal to 10.sup.-5 .mu.Ci per cc, which is the MPC level for Xe-133 established by the federal government. Accordingly, it can be seen that a Geiger tube count of approximately 18,000 is equivalent to 1 MPC-HOUR.
The radioactive gas monitor of the present invention is adapted to provide a direct readout in units of MPC-HOURS by accumulating the total count from a Geiger tube and continuously dividing the result by an appropriate calibration constant. In this manner, information relating to the total airborne radioactive exposure over an extended period of time is directly provided. Moreover, the present invention includes an internal pulse generator whose output is continuously subtracted from the count output of the Geiger tube to account for the normal background count rate of a Geiger tube due to ambient radiation. In addition, the present radioactive gas monitor provides means for measuring both the actual and subtracted background count rates and adjusting the subtracted background count rate, so that the instrument can be accurately calibrated for different ambient environments. More particularly, the present instrument includes a "TEST" position in which the actual background radiation, as well as the frequency of the internal pulse generator, can be alternatively measured on an analog meter and the frequency of the pulse generator adjusted until the two are equal. The preferred form of the present invention also includes a digital timer to provide an indication of the period over which the displayed measurement was taken.
Additional objects and advantages of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following set of drawings in which :